This invention relates to the storage of thermal energy, and particularly to a thermal energy storage material, a method of preparing this material, and a system incorporating the material.
Salt hydrates are known for their ability to store thermal energy. These salt hydrates absorb thermal energy as they melt or decompose and release heat as they revert to the solid hydrated crystalline form. Na.sub.2 SO.sub.4.10H.sub.2 O is the most commonly reported hydrate since it is relatively inexpensive and has high heat storage capacity. However, Na.sub.2 SO.sub.4.10H.sub.2 O has a number of undesirable characteristics that present difficulties when used in thermal energy storage systems. Although, ideally, the decomposition of Na.sub.2 SO.sub.4.10H.sub.2 O can be reversed on cooling, complete reversibility is difficult to achieve because rapid sedimentation of the decomposition product (anhydrous Na.sub.2 SO.sub.4) occurs. U.S. Pat. No. 3,986,969 to M. Telkes disclosed the use of thickening agents to reduce this sedimentation and improve reversibility. D. R. Biswas, "Thermal Energy Storage Using Sodium Sulfate Decahydrate and Water," Solar Energy 19, 99 (1977), discloses the use of excess water. Excess water ensures complete reversibility but the energy capacity of the system is reduced and the heat delivery occurs at a steadily declining temperature. In static systems, supercooling is reported and heat acceptance and delivery rates vary with the state of charge.